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TRACKS IN THE UPPER CRETACEOUS OF THE RATON BASIN POSSIBLY SHOW TYRANNOSAURID RISING FROM A PRONE POSITION

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  • New Mexico Museum of Natural History and Science

Abstract and Figures

After description of the first known footprint attributed to Tyrannosaurus rex by Lockley and Hunt in 1994, the lead author (TC) began a systematic search for more tracks of this giant theropod in the Raton Basin. During parts of eight field seasons in the mid and late 1990s and early 2000s, two finds were made and interpreted to be tracks of large tyrannosaurids very similar in size to the one that made the original footprint. The material, which is preserved as convex hyporelief, consists of a tridactyl right pes print lacking a hallux impression coming from a locality near Ludlow, Colorado, and a track array, here interpreted as a convex hyporelief left pes print with a single toe impression, likely that of a hallux, and a pair of parallel convex hyporelief forearm prints with partial hand impressions from Cimarron, New Mexico. Near the track array an "L"-shaped impression in convex hyporelief is interpreted to have been the impression of carrion or a prey item. Like the original material of Tyrannosauripus pillmorei, these traces are present in the Upper Cretaceous lower coal zone of the Raton Formation. The track array is hypothesized to have been made by a prone adult T. rex, rising from a quadrupedal position. In standing up, the dinosaur stepped forward and made the left pes print, while at nearly the same time it made the forearm prints as it pushed down with its forearms and wrists to facilitate its rise to a standing, walking, or running posture.
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Lucas, S. G., Hunt, A. P. & Lichtig, A. J., 2021, Fossil Record 7. New Mexico Museum of Natural History and Science Bulletin 82.
TRACKS IN THE UPPER CRETACEOUS OF THE RATON BASIN POSSIBLY
SHOW TYRANNOSAURID RISING FROM A PRONE POSITION
T CANEER1, TOM MOKLESTAD2 and SPENCER G. LUCAS3
1Friends of Dinosaur Ridge, 16831 W. Alameda Parkway, Morrison, CO 80465†;
2Castle Rock Historical Society and Museum, 420 Elbert St., Castle Rock, CO 80104;
3New Mexico Museum of Natural History, 1801 Mountain Rd. NW, Albuquerque, NM 87104, email: spencer.lucas@state.nm.us
Abstract—After description of the rst known footprint attributed to Tyrannosaurus rex by Lockley
and Hunt in 1994, the lead author (TC) began a systematic search for more tracks of this giant theropod
in the Raton Basin. During parts of eight eld seasons in the mid and late 1990s and early 2000s, two
nds were made and interpreted to be tracks of large tyrannosaurids very similar in size to the one
that made the original footprint. The material, which is preserved as convex hyporelief, consists of a
tridactyl right pes print lacking a hallux impression coming from a locality near Ludlow, Colorado, and
a track array, here interpreted as a convex hyporelief left pes print with a single toe impression, likely
that of a hallux, and a pair of parallel convex hyporelief forearm prints with partial hand impressions
from Cimarron, New Mexico. Near the track array an “L”-shaped impression in convex hyporelief
is interpreted to have been the impression of carrion or a prey item. Like the original material of
Tyrannosauripus pillmorei, these traces are present in the Upper Cretaceous lower coal zone of the
Raton Formation. The track array is hypothesized to have been made by a prone adult T. rex, rising
from a quadrupedal position. In standing up, the dinosaur stepped forward and made the left pes print,
while at nearly the same time it made the forearm prints as it pushed down with its forearms and wrists
to facilitate its rise to a standing, walking, or running posture.
INTRODUCTION
Fossilized dinosaur tracks provide a method for
understanding the activity and behavior of dinosaurs, and,
therefore, they may be thought of as “fossilized” activity. The
possible occurrence of a set of Tyrannosaurus rex-like prints
reported here that may be attributed to a T. rex rising from a
prone position is the rst report of any tyrannosaur showing
this behavior. Following the discovery in 1995, it was only
the second known T. rex track. This is also the rst report of
a pair of possible forearm prints with partial handprints of a
tyrannosaurid. The project that is the basis of this report was
undertaken in 1994 in an eort to nd T. rex tracks additional
to the one documented by Lockley and Hunt (1994), beginning
with determining where and how to conduct the search.
Subsequent interpretations of large tyrannosaurid pes prints
include: single footprints (Caneer, 1999; Chamberlain, 2007;
Fair et al. 2008; Manning et al., 2008; Lockley et al., 2011;
Lockley and Tempel, 2014, a single trackway (Perkins, 2016);
and multiple parallel trackways, suggesting gregarious behavior
among certain large tyrannosaurids in the Late Cretaceous of
Wyoming (Larson, 2003) and of northeastern British Columbia
(McCrea et al., 2014). The rst descriptions of dinosaurian sitting
postures (Lull, 1904, gs. 26, 28) are from a tridactyl, bipedal
herbivorous? track maker/s from the Late Triassic-Early Jurassic
of the northeastern U. S. Illustrated, paired manus and pes prints
together with a medially located and oriented impression of the
ischium were associated with tail drag marks. At the St. George
Dinosaur Discovery track site in Utah, symmetrically positioned
manus prints of a sitting Dilophosaurus-like theropod from
the latest Triassic have been documented. From anterior to
posterior, a six-point stance consisting of the aforementioned
manus prints, two opposed pes prints, an impression of the tip
of the ischium and a tail impression is evidence of a resting
posture described as crouching (Milner et al., 2009, gs. 4,7).
This stance occurs roughly midway in a much longer trackway,
so that this is evidence of a non-avian theropod rising from a
sitting position by stepping forward.
A skeletal adult theropod, Troodon formosus, about 1 m
tall at the hip, has been reported as associated with a clutch of
fossil eggs from the Late Cretaceous of Montana (Varricchio et
al., 1997). It has been depicted in a sitting posture on the eggs
in a scraped nest (Horner, 2000, g. 3). Partial skeletons of
multiple species of similarly small adult oviraptors have been
found positioned on traces of their own nests from the Late
Cretaceous of Mongolia and China (Osborn, 1924; Norell et al.,
1995; Dong and Currie, 1996; Clark et al., 1999; Fanti et al.,
2012; Bi and Xu, 2017; Norell et al., 2018). These and other
oviraptors left nesting traces dened by eggs in 2-4 stacked
near-circular concentric rings ranging 0.6-.8 m in diameter with
circular interior openings ranging 0.2-.3 m in diameter centered
within the rings and often slightly to signicantly raised (Yang
et al., 2019). Thought to have been rapidly buried in catastrophic
dune collapse events, the aforementioned single bodies of adult
specimens were found in the centers of their nests, suggesting
these were their original positions.
Without the benet of footprints, the oviraptor nesting
posture has been described as “crouching” (Fanti et al., 2012),
“sitting” (Norell et al., 2018) and “oviposition” (Yang et al.,
2019). With the abdominal region directly on the center of the
nest, with the arms extended laterally and exed 90° caudally
at the elbow along the circumference of the nest, a prone
posture is illustrated by Clark et al. (1999, g. 13). Without
their arms extended cranially, these animals may have been at a
disadvantage when trying to use their arms to facilitate standing
up to overcome or escape burial.
Currently, “resting” seems to be used in a couple of ways.
In a general sense it is used to describe one of two states, “not
in motion” as opposed to “in motion.” Thus, “resting” means
“at rest” or “not moving about,” as used by Milner et al. (2009),
for example. Various theropod postures can be associated with
being at rest, including standing after rising to fully erect,
standing normally, standing on one foot while scraping with
the other, crouching, oviposition, sitting, or prostrate (prone).
Although “resting” also may have been used specically by
some for “resting up,” it has notably been used together with
“sleeping” when describing the posture of a troodontid skeleton
from China with the skull under an arm in a stereotypical, bird-
like sleeping posture (Xu and Norell, 2004). Clearly, the arms
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of Tyrannosaurus rex are too short for such a sleeping posture,
leaving open the possibility that T. rex slept in a prone position,
a posture supported by interpretations of data presented here.
GEOLOGIC SETTING
The Raton Basin trends south from Walsenburg, Colorado
to Cimarron, New Mexico. It was chosen because the rst
Tyrannosaurus rex track previously reported (Lockley and Hunt,
1994) is located there (Fig. 1A). Because the Raton Basin is
about 160 km long and up to 80 km wide, the search area had to
be narrowed considerably. Since T. rex was a latest Cretaceous
animal, its tracks were anticipated in the uppermost Cretaceous,
nonmarine strata exposed at the surface. These factors limited
the search to the following three stratigraphic units: lower coal
zone of the Raton Formation; Vermejo Formation; and Trinidad
Sandstone (Fig. 1B). The lower coal zone of the Raton Formation
and the Vermejo Formation are deposits of the coastal plain west
of the Interior seaway where river deltas and associated forests
and swamps were the sites of initiation of coal formation. The
Trinidad Sandstone is the shoreline deposit of the Interior seaway
that marks the regression of the seaway during late Campanian-
Maastrichtian time behind which the Vermejo and lower Raton
coastal plain deposits accumulated. The K-Pg boundary is at the
top of the lower coal zone of the Raton Formation, about 50 m
above its basal sandstone member, which was targeted in this
search.
MATERIALS AND METHODS
The rock units listed above consist largely of sandstone and
shale layers, where tracks can possibly be observed on either
the top or bottom surfaces. Tracks made in sand may eventually
be found in the basin on the top surfaces of the sandstone beds
(positive tracks). Although tracks made in mud occur here, it is
unlikely that tracks on the top surfaces of shale survive erosion
long enough after exposure to be observed. However, when a
track in mud was covered by a layer of sand, the sand lled the
track, forming a natural sandstone cast (negative track) resistant
to erosion and observable at the exposed base of the sandstone
bed.
Searching for tracks on bedding surfaces of in-place
sandstone beds in the Raton Basin is very dicult because the
top and bottom surfaces are usually not visible. The top surfaces
of potential track-bearing sandstone beds, which coincide with
the current ground surface (dip slopes), are usually covered with
soil, vegetation and/or rock debris and are obscured from view.
Exceptions to this are where top surfaces are exposed along
rivers, such as is the case at track sites near Glen Rose, Texas
and La Junta, Colorado. Top and bottom surfaces can sometimes
be seen along cli faces where shales have been eroded from
between the sandstone beds. However, these occurrences are
quite limited in the Raton Basin.
An initial reconnaissance in the Raton Basin yielded plant
impressions and many trace fossils made by animal activity (but
not by Tyrannosaurus rex). They all occurred as negative tracks
on the bottom of sandstone beds in the lower half of the lower
coal zone of the Raton Formation. These trace fossils appear to
be tracks and other traces made by ornithopod dinosaurs and
crocodilians, burrows, and possible droppings. Plant material is
also evident. Figure 2 shows what appears to be an ornithopod-
like track on the bottom of a lower coal zone sandstone boulder,
possibly made by a hadrosaur. This track is about 60 cm long
and 65 cm wide. Its dark red color indicates that the track has
concentrated iron oxide. Because all of the above tracks and
other traces occur as negative traces on the bottom of sandstone
blocks from this zone, tracks of T. rex are most likely to be found
as negative tracks on the bottoms of sandstone blocks from this
zone.
Under these conditions it was determined that the best
opportunity for nding tracks of Tyrannosaurus rex would be
to look at the accumulations of Upper Cretaceous nonmarine
sandstone exposed as boulders, which have fallen from the
walls of canyons that have been carved by rivers that ow out of
the Sangre de Cristo Mountains. These large sandstone blocks,
with potential track-bearing surfaces that measure up to 5 m on
each side, are present in great numbers below the clis. There
is a very good likelihood that fallen blocks have come to rest in
positions so that the potential track-bearing surfaces are exposed
for viewing.
TRACKS AND OTHER TRACES
After a considerable amount of searching, the following
two sites of probable T. rex tracks were found:
1. An isolated footprint (Fig. 3A) occurs on the bottom of
a sandstone block from the lower coal zone in Berwind Canyon
near Ludlow, Colorado, in the northern part of the Raton Basin.
It is approximately the same size and shape (Fig. 3B) and is from
the same geological formation as the rst veried Tyrannosaurus
rex track (Lockley and Hunt, 1995, p. 235). Both tracks have
three long, slender toe impressions of weight-bearing toes. Both
tracks are about 85 cm long and 55 cm wide at the posterior end
of the digit impressions. Whereas the previously published track
shows the trace of the hallux, the track from Berwind Canyon
FIGURE 1. A, Locations of specimens described in this study.
B, A simplied stratigraphic section, including the track-bearing
zone.
FIGURE 2. An apparent ornithopod-like track, possibly made
by a hadrosaur. Located in the New Mexico portion of the Raton
Basin. The red-stained track is approximately 65 cm long and
60 cm wide.
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of the Raton Basin in northeastern New Mexico. The following
is a detailed description, discussion and interpretation of these
latter traces.
Description of the Track Array
The track array consists of rather large bulging features on a
sizeable block of sandstone near the base of a slope northwest of
Cimarron, New Mexico (Figs. 4, 5A-B). A result of mass wasting
of an outcrop of a sandstone unit from the Upper Cretaceous
lower coal zone of the Raton Formation, the boulder fortuitously
rolled into a position where its original planar base with bulges
can be readily viewed.
The largest and lowermost feature lled an L-shaped
impression resulting in a reverse L-shaped bulge. With lengths
(L) measured from the apex, the longer limb is L=83 cm and W
(width) =31 cm, the shorter limb is L=64 cm and W= 24 cm.
Six smaller bulges in a tight grouping near the hammer
are more or less equidimensional. They are generally rounded,
including their edges on the outer edge of the tight grouping
of the lower four (Fig. 5A). The three largest are well-rounded
structures that bulge from the bedding plane as shown in low
angle views in Figures 5B and 6B. By contrast, the interior
edges of the bulges in the cluster of four are straight, and, where
they meet in the center of the group, they are angular, forming
right angle corners (Figs. 5A and 6A). The long dimensions
(Lmax) and the heights (H) range from the smallest bulge at the
top of the group of six, Lmax=11.5 cm, H=2.5 cm to the largest
bulge at the bottom, Lmax=25.4 cm, H=12.7 cm. In general, the
trend is the bulges become broader and taller from the top to the
bottom of the group of six. Three rows of two and two columns
of three within the cluster of six form a trapezoidal shape with
the unequal parallel sides (bases) formed by the smallest row of
two and the largest row of two (Fig. 5A).
To the right of the reverse L-shaped bulge, two medium-
sized, elongated bulges are parallel, similarly sized and similarly
shaped (Fig. 5A). Both taper to sharp terminations pointing to
the cluster of six (Fig. 5A) and to the left (Fig. 6A, C). Both
widen gradually to the right in Figure 6C where the upper bulge
bifurcates into two nearly equal, short and narrow branches,
and the lower bulge also widens before it sharply narrows to a
single “branch” that is similar to the other two. The upper bulge
(Fig. 6C) has a sharp-crested medial ridge that extends from the
pointed left end to the point of bifurcation. The lower bulge also
has a sharp-crested ridge extending to the right from the left end.
Short of the midpoint the ridge ends, replaced by a very rough
surface that may represent a surface of breakage that extends to
FIGURE 3. A, Tyrannosauripus pillmorei reversed fossil
footprint on outcrop in Berwind Canyon in the Raton Basin near
Ludlow, CO. Two toes of the darkened track are visible on the
overhang; B, The track cast is L = 85 cm, W = 55 cm. Hammer
is 28 cm.
FIGURE 4. A fallen sandstone block with the “track array” in
the southern Raton Basin. The block is above the arrow. For
scale the boulder dimensions are: L (length) = 3.5 m, W (width)
= 2 m and T (thickness) = 1.5 m.
lacks this. However, the former is twice as deep (~23 cm).
This track is apparently not deep enough for the hallux to have
touched the ground. Except for a deep penetration of the foot
into the sediment, such as is the case for the veried T. rex track,
it is likely that the hallux would not have made an impression,
particularly from a walking posture. Also, there is signicant
erosion where the hallux of this track may have left a trace.
2. A “track array” consisting of three possible tracks, is
interpreted here as one footprint and two forearm/hand prints
attributed to Tyrannosaurus rex; it was found in the southern part
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FIGURE 5. The “track array” on the boulder of Figure 4. A, The underside of the sandstone block standing on end has nine
prominent bulges colored black with a mixture of powdered charcoal and water. Rock hammer is 28 cm long; B, The track array in
prole. View is along the track surface from the right side of the block as seen in Figure 4. The rock hammer is 28 cm long. Note the
hammer is in a slightly dierent position, hanging from the high point of the boulder, left of its position in Figure 4.
the right to the narrowing. A piece or pieces broken o may have
had a continuation of the medial ridge and possibly a second
branch that would have been part of a bifurcation. The lower
bulge is obviously longer. The upper bulge is L=25 cm, W=10
cm, H=4.5 cm and the lower bulge is L=28 cm, W=9 cm and
H=4.5 cm, where L is length, W is maximum width and H is
maximum height.
Interpretation of the Track Array
The following interpretation that the track maker was
a Tyrannosaurus rex is based on the trapezoidal group of six
darkened impressions near the top of the sandstone block in
Figure 5A, plus the two short parallel ridges that are on the lower
right. The reversed L-shaped bulge to the left of the parallel
ridges will be discussed separately.
The rst interpretive task was to determine the likelihood
of the bulges having been made by either vegetation, geological
process, or animal activity. The shape and pattern of the array
make it unlikely that it was made by vegetation. For instance,
the three-dimensional aspect of the array does not suggest
leaves or leaf-like structures that commonly produce very
at impressions. In the trapezoidal cluster, lack of linear,
cylindrical and branching shapes and patterns does not support
identication of the impressions as from tree limbs, trunks or
roots. Instead, rounded, nearly equidimensional shapes are more
similar to those of plant structures like seed pods, fruit or tubers.
But, tubers would have their long dimensions more vertical
with complete burial rather than the observed partial burial.
Signicant imbedding of seed pods or fruits could result from
a fall from a signicant height or possibly from being stepped
on by a large animal, but evidence of attening or other damage
from impact or trampling is not apparent. There is attening
internal to the cluster of four where the bulges seem to have
been squeezed laterally towards each other, suggesting that
these bulges represent somewhat soft, resilient structures that
survived their imbedding without obvious damage.
The two short linear parallel impressions are not consistent
with cylinders, a common shape for branches and roots. Instead,
the shapes are moderately complex and generally bilaterally
symmetrical to each other, suggesting an organic rather than
sedimentological origin. A specic fossil plant from the Upper
Cretaceous of the Raton Basin with a structure/s that could have
made these impressions has not been identied.
Although the “L”-shaped impression has the general look
of a log impression with the slimmer segment branching from
the broader one, there are no bark impressions, no branch or root
collar impressions in the vertex of the “L,” and no “Y” shape
that would prove branching. The imbedding of a short log is also
problematic. Lack of coaly remnants present elsewhere in the
lower coal zone of the Raton Formation also does not support
vegetation as a source of the bulges.
Relative to geological processes, scour and ll is not a
likely process for making these track-like features because
pebbles that occur in the tracks are concentrated on the edges
of the impressions. If the pebbles were washed into a scour, or
ute, they would have settled at the bottom of the depression,
not along the upper sides. Also, the sharpness and detail of some
of the features of the array are not consistent with scour and
ll. Load casting is another geological process to consider. Load
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casts are irregular protuberances of sand or gravel that extend
downward into ner-grained, softer material such as mud, wet
clay, peat, marl, etc. The array is not likely to be associated with
load casting because load casts would be inconsistent with the
symmetry and sharpness of the features of the array.
With plant fossils and sedimentary structures seeming
unlikely, the bilateral symmetry of both the group of four in the
trapezoidal cluster and the pair of linear parallel impressions
suggests an organic origin. Hence, the bulges can be analyzed as
possible traces of large, heavy animals, presumably vertebrates.
During the latest Cretaceous in the Raton Basin, these would
have been dinosaurs, including titanosaurs, ceratopsians,
hadrosaurs, and tyrannosaurs. By far, the most common
recognized impressions left by dinosaurs are footprints. These are
very convincing when toe impressions are part of the structure,
particularly when similar structures occur in abundance and/or
repeat in a pattern, such as in a trackway. The most common
postures interpreted from dinosaur footprints appear to be
walking and, less frequently, running. Rare examples of other
postures include swimming/deep-water wading (Moklestad et
al., 2018, gs. 4-5) and standing on one foot while scraping with
the other, which can be inferred from Lockley et al (2016, g.
3b-c; 2018a, g. 10d). When distinct toe impressions are present,
descriptions of isolated single pes prints (Lockley and Hunt,
1994, Lockley, 2012) or a single manus-pes pair (Mossbrucker
et al, 2008, g. 2) have been made.
FIGURE 6. A, Cast of the track array. To the left of the hammer handle are three pads that were behind the toes that pointed to the
right. To the left of the hammer head are from left to right, reversed impressions of the hallux pad and the hallux claw. To the right of
the hammer are two forearm-with-hand tracks pointing to the right. Hammer is 28 cm; B, Cast of the footprint from the track array.
Low angle view illustrates increase in the height of bulges (sole pads) from the posterior end to the anterior end of the track. Bulge
next to pick has sharp edge attributable to the tip of a hallux claw; C, Cast of two tracks within the track array representing forearms
with hands (metacarpals only) pointing to the right. The hammer is 28 cm long; D, Outline drawing in red by TM of a cast of the
track array footprint by TC superimposed at the same scale on a photograph by SGL of Tyrannosauripus pillmorei, found by Chuck
Pillmore on a fallen boulder near Cimarron, New Mexico, and later described by Lockley and Hunt (1994). Hammer is 28 cm.
34
Stand-alone dinosaur footprints without toe impressions
have apparently largely gone unrecognized or unreported.
Images of small circular features (diameter ≈10 cm) without toe
impressions at the Duncan Road theropod scrape site (Lockley
et al., 2016, g. 4a) and the Cherryvale dinosaur and turtle track
site (Lockley et al., 2018b, g. 10) have been published but not
interpreted. At Duncan Road, four unmentioned circular tracks
are associated with a scrape consisting of three troughs made
by a small theropod about one meter tall at the hip. Tracks 2R
and 4R are within the posterior portion of the right trough, while
tracks 2L and 4L are at the posterior edges of the center and left
troughs, respectively (Fig. 7). At Cherryvale, about 17 features
that look similar to the four at Duncan Road are grouped on a
single slab. They were described as “circular to sub-circular pit-
like features” and were presented as of unknown origin (Lockley
et al., 2018b). Unlike at Duncan Road, a linkage to known
theropod activity is not apparent. On the other hand, these tracks
appear to have been made by a biped because the tracks occur
in pairs with feet opposed (n = 5) and feet staggered (n = 2). A
trackway of bipedal stances progressing sideways to the left and
then backwards can be interpreted. At both sites these adactyl
tracks appear to have been made when small theropods rose to
more or less fully upright while stepping backwards with their
weight (50-75 kg?) mostly on the soles of their feet and well o
of their toes (Moklestad, unpublished). Unfortunately, details of
the geometry of the sole pads within these sole prints are not
apparent.
The trapezoidal cluster of bulges within the track array is
within a range of sizes that has been observed for footprints of
large dinosaurs. Although the cluster or its mirror image does
not repeat, a succeeding print could have been made beyond the
limits of the sandstone block. Also, the cluster lacks obvious
toe impressions, specically impressions of toes bearing weight
during walking so that the cluster may represent a posture other
than walking. The existence of toes is suggested by the pattern
of the three largest bulges (Fig. 5A) that may represent pads on
metatarsals each proximal to a toe, making the trapezoidal cluster
primarily a sole print. Contextual evidence for the orientation of
the footprint and direction of its anterior end includes:
· 1. The three largest bulges are in a triad (Fig. 5A) with an
axis of bilateral symmetry through the center or lead bulge. If
extended toward the L-structure, it indicates the position and
orientation of an unweighted middle toe and thereby the anterior
of the foot.
· 2. The proposed forearm-wrist-hand impressions (to be
discussed) include a bifurcation of the right forearm print,
which, if made by the manus, indicates the distal or anterior end
of that impression. It points in roughly the same direction as
the proposed footprint. In addition to evidence indicating the
anterior end of the footprint, the proposed hallux impression (to
be discussed), particularly the attachment, is near the posterior
end of the proposed footprint, a position observed on other
theropod footprints, including Tyrannosauripus pillmorei. The
asymmetry of the trapezoidal cluster resulting from the hallux
impression provides a means for determining that the cluster is
a left footprint.
In contrast to the symmetrical, tightly-grouped cluster of
four rounded structures interpreted here as sole pad impressions,
the two smaller structures to the left of the hammer in Fig. 6B
disrupt the otherwise symmetrical pattern and are separated
noticeably from the cluster of four and each other. Separation
from the main cluster made by the sole of a foot could indicate a
protrusion, logically a toe or toes, possibly a hallux. The hallux
of Tyrannosaurus rex has three segments of roughly equal
length (Paul, 2016), a proximal tarsal, a distal tarsal and a claw,
so that the small bulge may represent the proximal part of the
proximal tarsal or more, and the larger bulge may be from the
distal part of the claw or more. The sharp distal edge of the large
bulge normal to the bedding surface (Fig. 6B) is consistent with
the tip of a sharp claw being pressed vertically down into the
substrate. Hence, the gap between the inferred hallux bulges
may have resulted from a curved toe, concave downward as a
result of joints in the toe and curvature of the claw. The size,
shape and point of attachment of the hallux is similar to that of
Tyrannosauripus pillmorei, as identied by Lockley and Hunt
(1994, 1995). The more cranial orientation of the hallux, together
with curvature in a vertical plane rather than a horizontal plane,
may be related to posture – a rising from a prone position as
opposed to a walking posture.
Titanosaurs (e.g., Alamosaurus) and ceratopsians (e.g.,
Triceratops) were ruled out as the track maker because they had
feet with more than three weight-bearing toes and no hallux claw.
Hadrosaurs (duckbills, e.g. Edmontosaurus) and tyrannosaurs
had feet with only three weight-bearing toes, but hadrosaurs
can be ruled out because they did not have a distinctive hallux
or a hand with only two ngers. Large tyrannosaurs, including
Tyrannosaurus rex, had three weight-bearing toes, a hallux, a
short forearm and a didactyl manus. Therefore, the evidence of
unique body parts together with the size, shape and conguration
of the track array suggests a large tyrannosaur. At the time the
track array was found, the location of the only veried T. rex
track (Tyrannosauripus pillmorei) was not available except
that it was found in the Raton Basin. A few months after the
subject track array was found, it was learned that it is located
within 3.2 km of the type specimen of T. pillmorei and from the
same formation. So, similarities with the maker of T. pillmorei,
including a hallux, three weight-bearing toes, size, shape,
FIGURE 7. An outline drawing of a theropod scrape with three
troughs and four related adactyl tracks from the Cretaceous
Naturita Formation at the Duncan Road scrape site in western
Colorado. Near-circular impressions, tracks 2L and 4L,
followed the last swipe in the left scrape and the far left scrape,
respectively. X’s mark inferred foot positions based on a model
of theropod scraping whereby the theropod maintained its
position and orientation at the point of origin while scraping,
normally at the midpoints of full length scrapes. However in this
unusual case the scraper stepped backward twice from the point
of origin onto its “heels,” causing it at the same time to rise to
a more fully upright posture. Initially it stood on one foot at 1L
and scraped the right trough. Then it stood on the right foot at
R1 while scraping the center trough. Then it stepped back to
2L, then 2R. After surveying the situation from skewed stance
2R-2L it chose to scrape again and stepped forward to 3R and
stood there on one foot while scraping the far left trough. Then
it stepped backward to 4L and then 4R as it rose for another
look from a square stance. Then it abandoned the scrape without
completing a nest. Foot position 3L was not necessarily used.
Sketch based on an unannotated photogrammetric image from
Lockley et al. (2016, g. 4a).
35
geologic age (Pillmore and Fleming, 1990) and stratigraphic
position in the Raton Formation close to the K-Pg boundary
(Lockley and Hunt, 1995), are supportive of the likelihood that
the track array was made by a T. rex.
If a Tyrannosaurus rex were in a prone position, it is
reasonable that it would use its forearms and hands to help
support its upper body to some extent, especially if its arms
were strong. If so, the forearms would have made impressions in
the sediment, given a suciently soft substrate. Support of this
scenario came when Carpenter and Smith (2001) concluded the
forearms of T.rex were strong, and Lipkin and Carpenter (2008)
followed up by documenting evidence of signicant strain in
the form of healed stress fractures and other healed trauma in
the pectoral girdles of multiple specimens of T. rex, suggesting
injuries from a muscular upper body of this predator as it fought
with its arms to restrain violently strong, struggling and resisting
prey.
These nearly identical arm? tracks have an axis of bilateral
symmetry corresponding roughly with the handle of the hammer
in Fig. 6C. The left forearm/hand track (below the hammer) has
been damaged. Impact pits approximately the size of the head of
a rock hammer like the one shown can be seen on this track. This
should not be unexpected because the block on which the tracks
occur is located at a place where many people have probably
seen it. The undisturbed right forearm/hand track (above the
hammer) consists of a ridge (forearm) that increases in height
(reecting an increase in depth) from proximal to distal until
it reaches the wrist impression, at which point it branches into
two sub-ridges. These two sub-ridges were apparently made by
the metacarpus, which only had two metacarpals, and therefore
two ngers. We suggest that the tracks are deepest at the wrists
because the T. rex was pushing down and forward at the wrist
in order to assist it in rising from a prone position. This activity
would cause the ngers to be above the ground, and they would,
therefore, not leave impressions.
In order to further conrm that these tracks were made
by the forearms and metacarpi of a Tyrannosaurus rex, a
comparison was made with the dimensions of the forearms and
metacarpals of the T. rex skeleton found by Kathy Wankel. Her
T. rex, illustrated in Horner and Lessem (1993), was chosen for
comparison because it was, at that time, the only T. rex skeleton
found that had a complete set of arms from the shoulders to the
ngers. A comparison did indeed show that the dimensions of
the forearms and hands of the track array were comparable with
those of Kathy Wankel’s T. rex skeleton (John Horner, personal
communication).
One diculty with the interpretation that the track maker
was a Tyrannosaurus rex is that the pes made three somewhat
circular traces, not three long slender traces that would be
expected from a walking dinosaur. “Walking” is the key word.
The track maker was not walking, it was probably rising from a
prone position. If the T. rex moved its left foot forward and dug
in its left heel (at the back of the toes) and pushed down with its
left leg and forward with its right leg in order to stabilize itself
and push its body to the rear while rising from a prone position,
the front of the toes of the left foot would be above the ground
(Fig. 8B). The prole of the track array in Figure 5B shows that
the thrust of the left foot was forward (toward the bottom of
Fig. 5B), which would cause the body to move back rather than
forward. Only the pads at the back of the toes where the toes
join the metatarsal (vertical) bones would make tracks, and they
would be short and somewhat circular, as is the footprint in the
track array. This movement would also explain why the footprint
and the forearm/hand prints are so close together.
Another factor that supports the interpretation that the track
array was made by a Tyrannosaurus rex is the similarity of the
size and shape of the track array footprint to comparable parts
of Tyrannosauripus pillmorei. Both are reverse left footprints.
Fig. 6D is an outline of the track array footprint (red lines)
superimposed over a photo of T. pillmorei at the same scale. The
track array’s footprint ts well with the back of the toes of T.
pillmorei and the location of the hallux pad. Also, the lengths of
the hallux and hallux claws are comparable. The positions of the
hallux claws are dierent, but this could result from the mobility
of the hallux together with dierences in weight distribution on
the left foot between standing up in the case of the track array
and walking in the case of T. pillmorei.
Track Maker Stance and Movement
The drawings of a Tyrannosaurus rex in Figs. 8A-C are
modeled after a sculpture by Matt Smith in Horner and Lessem
(1993, p. 96-97). In Fig. 8A, T. rex is shown in a prone position
supported by its forearms as it was balanced on the front of the
pubic boot (a wide expansion at the end of the pubis in some
dinosaurs). The vertical lines show the location of the front and
back boundaries of the track-bearing surface of the sandstone
block on which the track array resides (Fig. 5A).
In the track-making sequence, the rst tracks were made by
forearms/hands (Fig. 8A). The hind feet were beyond the limits
of the track-bearing sandstone block. Later, the Tyrannosaurus
rex rose by both advancing its left foot forward and pushing o
with its wrists and forearms followed by digging in its left heel
to maintain stability as it moved its body and center of gravity
rearward (Fig. 8B). By digging in its left heel, only the pads
under the back of the toes, plus the pad under the attachment of
the hallux and the hallux claw, would make tracks. The front of
the toes would be above the ground and would not make tracks
FIGURE 8. Sketches of postures of Tyrannosaurus rex by the
late Paul Koroshetz. A, Resting in a prone position balanced on
the forearms and the front of the pubic boot. Note the positions
of the forearm impressions and the right foot do not change in
Fig. 8B; B, Rising from the prone position of Fig. 8A, resulting
in the track array. Note the body moved rearward by about the
length of the skull after the forearms pushed downward and
the left leg reached forward and straightened; C, Hypothetical
sitting posture. Weight on both feet and the pubic boot. Bones
are exposed to illustrate the pubic boot (forward) and the
ischium (rearward). Tail shown as a counter weight, but could
have served as a support beam against the surface of the ground.
36
there. This activity would result in a conguration like that in the
cast (Fig. 6A). Also, this posture (Fig. 8B) provides a mechanism
for making a hallux impression as part of a relatively shallow
track. Alternatively, the proximity of the pads to the L-shaped
reversed impression (Fig. 5A) suggests the T. rex may have
stepped on it with its toes, propping the toes up, driving the three
foot pads down, and possibly straining the muscles of the foot
so that the left sole pad was stretched and somewhat elongated
(Figs. 5A, 6B) and the hallux pointed more forward than in
Tyrannosauripus pillmorei (Fig. 6D). This would account for
the lack of toe prints and explain how a log or piece of the leg or
arm of prey/carrion became imbedded. Presumably the carrion
would have somehow been removed, possibly by a scavenger/s
prior to deposition of sand into the impression.
Because the track array contains tracks likely made by the
forearms and hands, it follows that they had a useful purpose,
contrary to a widely held belief that Tyrannosaurus rex arms
were too small to be useful. One question arises relative to
whether or not the arms would be strong enough to be of use
for a T. rex in rising from a prone position. Would it be able to
straighten its arm at the elbow far enough to be of use in rising?
It could not rotate its forearm about the elbow more than 90°
from its upper arm. Even if T. rex did not have signicant upper
arm strength, most of this strength was probably concentrated
in contracting the muscles that would pull the forearm up, not
down, as would be most useful in rising. However, the strong
shoulders could have been useful to assist with rising.
The use of its forearms in rising may have been more a matter
of balance than strength. When standing still, a theropod’s center
of gravity was slightly ahead of its hip socket (Paul, 1988). When
on all fours, the center of gravity of a Tyrannosaurus rex would
have shifted forward toward the front of the pubic boot. This
means that its weight could have been balanced on the front of
the pubic boot when it was on all fours. Under those conditions
it would have taken less arm strength to assist rising. If for no
other reason, the use of the arms could have kept it from falling
on its face while feeding, stalking, sleeping or in some other
quadrupedal activity. Feeding is a good possibility because the
reverse “L”-shaped ridge beside the forearms/hands tracks (Fig.
4A) could have been made by prey or carrion on which the T.
rex was feeding.
The enlarged pubic boot of Tyrannosaurus rex may have
evolved to compensate for the smaller arms in supporting its
upper body weight when it was in a prone position. Perhaps the
pubic boot served in much the same manner as the rocker serves
a rocking horse. That is, the center of gravity of a rocking horse,
or a T. rex, stays directly above the point of contact of the rocker
with the ground, when they are in a balanced position (Fig. 8A).
By being in a balanced position on the front of the pubic boot
when T. rex was in a prone position, it could quickly return to a
bipedal stance when necessary. When resting, T. rex may have
assumed a position as shown in Fig. 8C. When its body was
tilted up at the proper angle, the long axis of the pubic boot
made contact with the ground, as possibly would the end of the
ischium. This would probably be the most comfortable resting
position.
Waiting in ambush for prey, Tyrannosaurus rex may have
waited in a prone position. It balanced on the forward part of
its pubic boot, head down, hands down and its center of gravity
slightly in front of its forward foot – ready to spring into action.
It resembled a sprinter poised in the starting blocks, ready to
bolt forward. On the other hand it may have assumed a similar
position as a scavenger feeding on carrion.
CONCLUSIONS
The structures described here were collected in stratigraphic
and geographic proximity to bona de tracks of Tyrannosaurus
rex. However, these structures are unique in their shape and
not readily assigned to any known ichnotaxon. We interpret
these structures as biogenic in origin, and they most readily
t the hypothesis that they are recording the standing up from
a prone position of a large tyrannosaurid theropod. Given the
unique nature of these structures, and our limited knowledge of
tyrannosaurid behavior, this is an interpretation consistent with
the data. Thus, we oer this interpretation as a possibility to be
evaluated and further tested by additional trace and body fossil
knowledge of tyrannosaurids.
ACKNOWLEDGMENTS
The junior authors of this paper acknowledge the tenacity
and perspicacity of deceased senior author T Caneer in nding
and interpreting the traces described here. Adrian Hunt and
Hendrik Klein provided constructive reviews of an earlier
version of the manuscript. Ken Carpenter suggested a pertinent
citation of pioneering work by R. S. Lull. Keith Berry attempted
to relocate the tridactyl pes print in a rugged part of Berwind
Canyon. Andrew Moklestad assisted with exhibits. Cathy Smith,
daughter of T Caneer, generously provided access to her father’s
drafts, notes and photos as well as casts he made in the eld at
both fossil sites reviewed herein.
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The articulated postcranial skeleton of an oviraptorid dinosaur (Theropoda, Coelurosauria) from the late Cretaceous Djadokhta Formation of Ukhaa Tolgod, Mongolia, is preserved overlying a nest. The eggs are similar in size, shape, and ornamentation to another egg from this locality in which an oviraptorid embryo is preserved, suggesting that the nest is of the same species as the adult skeleton overlying it and was parented by the adult. The lack of a skull precludes specific identification, but in several features the specimen is more similar to Oviraptor than to other oviraptorids. The ventral part of the thorax is exceptionally well preserved and provides evidence for other avian features that were previously unreported in oviraptorids, including the articulation of the first three thoracic ribs with the costal margin of the sternum and the presence of a single, ossified ventral segment in each rib as well as ossified uncinate processes associated with the thoracic ribs. Remnants of keratinous sheaths are preserved with four of the manal claws, and the bony and keratinous claws were as strongly curved as the manal claws of Archaeopteryx and the pedal claws of modern climbing birds. The skeleton is positioned over the center of the nest, with its limbs arranged symmetrically on either side and its arms spread out around the nest perimeter. This is one of four known oviraptorid skeletons preserved on nests of this type of egg, comprising 23.5% of the 17 oviraptorid skeletons collected from the Djadokhta Formation before 1996. The lack of disturbance to the nest and skeleton indicate that the specimen is preserved in the position in which the adult died. Its posture is the same as that commonly taken only by birds among tetrapods that brood their nest, and its close proximity to the eggs indicates that the nest was not covered, indicating that the behavior of sitting on open nests in this posture evolved before the most recent common ancestor of modern birds
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Previously-unknown large scale scrapes attributed to Cretaceous theropod dinosaurs from the Naturita Formation (formerly the Dakota Sandstone) of western Colorado were recently named as Ostendichnus bilobatus and interpreted as evidence of “nest scrape display,” a type of courtship behavior previously known only in extant avians. However, comparatively little is known of the morphology, distribution and preservation potential of either modern or ancient nest scrapes. Further study of the initially described samples combined with new discoveries brings the total number of known in Colorado sites to five, one with two scrape-bearing levels. Combined, these sites preserve a total of more than 100 recognizable scrapes from all these sites. We also identify the first O. bilobatus-like scrape from the Cretaceous of Canada. Although variable, a majority of the large sample of Colorado scrapes have the diagnostic characteristics of O. bilobatus, with two lateral troughs separated by a median ridge, and are sufficiently distinct to allow measurement of salient features such as scrape size, depth, and median ridge and average trough width. These provide data which indicate that theropod nest scrapes range from ∼50 to ∼200 cm in length and up to ∼25 cm in depth, presumably indicate dinosaurs of different sizes, and variable time and energy spent in creation of individual scrapes. Scrape orientations are highly variable. Three of the sites occur at about the same stratigraphic level, although they are ∼3.0–∼6.0 km apart, suggesting that display arena sites may have been large, involving many dinosaurs and repeat activity in sequential breeding seasons. High-precision U–Pb zircon analyses by the CA-ID-TIMS method from a volcanic ash bed above the scrape bearing levels at Roubideau Creek (Colorado) yielded a weighted mean ²⁰⁶Pb/²³⁸U date of 97.689 ± 0.037 Ma (2σ internal error) and indicate a Cenomanian age for O. bilobatus scrapes in western Colorado.